One common application for thermal sensors is in thermal (infrared) imaging devices such as night vision equipment. One such class of thermal imaging devices includes a focal plane array of ferroelectric thermal sensor elements coupled to an integrated circuit substrate with a corresponding array of contact pads. The thermal sensors define the picture elements (or pixels) of the thermal image.
Each thermal sensor includes a ferroelectric (or pyroelectric) element, which may be a portion of a ferroelectric slab, formed from a ferroelectric material that exhibits a state of electrical polarization that depends upon temperature (such as in response to thermal radiation). On opposing surfaces of the ferroelectric element are disposed an infrared absorber electrode and a sensor signal electrode. A ferroelectric transducer element is formed by the infrared absorber electrode and sensor signal electrodes, which constitute capacitive plates, and the ferroelectric material, which constitutes a dielectric.
The thermal-image sensor signal appearing on the sensor signal electrode depends upon the capacitance of the ferroelectric transducer element, which in turn depends upon incident infrared radiation (temperature). The sensor signals from the thermal sensors in the focal plane array are coupled to an integrated circuit substrate which provides image processing, with each thermal sensor (i.e., each sensor signal electrode) being electrically coupled to a corresponding contact pad.
To maximize thermal response and ensure thermal image accuracy, each ferroelectric thermal sensor of the focal plane array must be thermally isolated from the surrounding focal plane structure, and from the integrated circuit substrate to insure that the associated transducer capacitance accurately represents incident infrared radiation. Thermal-isolation intermediate structures must be disposed between the focal plane array and the integrated circuit substrate to provide a bonding and sensor signal path interface that minimizes thermal diffusion.
The intermediate thermal isolation structure comprises two elements--a conductor element and a thermal isolation element. This general configuration for a thermal isolation structure can be represented by a thermal circuit with two parallel thermal current paths, one through the low-thermal-resistivity conductor and one through the high-thermal-resistivity thermal isolation structure. (See, for example, FIG. 2.) The design goal is to minimize the total thermal current through these two paths.
Several approaches have been used to provide a thermal-isolation intermediate structure for isolating a thermal sensor array from an underlying integrated circuit substrate. One approach is disclosed in U.S. Pat. No. 4,663,529 (Jenner), in which a square grid of channels form a corresponding grid of pillars that define thermal sensor elements. Each pillar or sensor includes a central bore that is coated with a conductive layer. The conductive bores are dimensioned to be less in diameter than corresponding electrode bumps disposed on an integrated circuit substrate, such that when the focal plane array of pillars is disposed over an integrated circuit substrate with a corresponding array of electrode bumps, the conductive bore of each thermal sensor rests on, and is electrically connected to, a corresponding electrode bump. A disadvantage of this architecture is that mating the array of conductive-bore pillars with the corresponding electrode bump array requires close tolerances and exact alignment. Another disadvantage of this architecture is that photoresist is used as the structural material for the pillars, which are therefore structurally fragile and susceptible to damage by solvents.
An alternative approach is disclosed in U.S. Pat. No. 4,143,269 (McCormick), assigned to Texas Instruments Incorporated, the assignee of this invention, where a thermal-isolation intermediate structure for a thermal sensor array uses conductive vias formed in a thermal isolation layer (polyimide) that covers an integrated circuit substrate. In this architecture, vias are formed in the thermal isolation layer, exposing contact pads on the circuit substrate. The sensor signal electrode for a thermal sensor is brought into contact with a corresponding conductive via, providing an electrical connection to the associated contact pad. A disadvantage of this architecture is that so much polyimide is present that total thermal resistance is relatively low. In addition, this architecture requires a relatively large number of process steps, thereby increasing costs.
Heretofore, mesa structures have not been used to thermally isolate the thermal sensors in a focal plane array from an underlying integrated circuit substrate. A mesa is a bump or pillar with a relatively small cross-sectional area that projects from a substrate. Typically, mesa structures are formed by photolithographic techniques (either etch or deposition processes). Mesa structures of materials other than polymer materials (such as polyimides) have been used in the fabrication of solid state devices for such applications as providing arrays of multiprobe contacts or spacers.
Accordingly, a need exists for an improved thermal-isolation intermediate structure that provides a bonding and sensor signal interface between a thermal sensor element and an underlying substrate. An advantageous structure would be capable of fabrication in a relatively few number of process steps.